This proposal is for a mentored research award to support Dr. Rosa Xicola's career development while she identifies the molecular etiology of uncommon phenotypes of hereditary cancer. Lynch syndrome (LS), is a multi-cancer syndrome caused by germline mutations in Mismatch Repair (MMR) genes that result in microsatellite unstable (MSI) tumors. Over half of presumed LS cases do not have germline mutations in MMR genes and these are referred to as Lynch-Like Syndrome (LLS). LLS patients are treated as LS patients and go through strict, invasive cancer surveillance. We hypothesize that there is a sizable group of colorectal cancer (CRC) patients with a genetically predisposing cancer condition due to germline variants in DNA repair genes that trigger genomic instability and consequently cancer development, with or without MSI. This could likely explain a number of the familial occurrences of multiple different tumors, such as breast and CRC. The goal of this proposal is to determine the extent to which mutations in DNA repair genes contribute to cancer development in patients with a strong family history of CRC or breast cancer. To accomplish this objective, this proposal pursues the following Specific Aims: 1) Determine the prevalence and functional implications of DNA repair loss of function variants in the cancer development of LLS patients. 2) Identify the tumor mutation landscape that triggers tumor development in LLS patients. 3) Identify DNA repair gene variants in a highly characterized group of young cancer patients with family history of cancer that lack known mutations in cancer predisposition genes. This proposal has the potential to have a significant clinical impact; establishing the genetic cause of undefined cancer syndromes could allow us to tailor both cancer prevention and treatment. Moreover, this proposal might identify the missing molecular link to explain familial co-segregation of the most common cancers, colorectal and breast. Through this award, Dr. Xicola will acquire and hone advanced skills in genetics, bioinformatics and biostatistics. She will also master the techniques required to engineer the genetic changes that she identifies into cell lines and investigate their biological effect on cancer development. The training and career development plan that has been created with her mentor, Dr. Joann Sweasy and co-mentor Dr. Allen Bale at Yale University along with her team of advisors, will support her independence by providing her with a well-defined expertise. After this award, Dr. Xicola will expand her research program as an independent investigator addressing the unanswered questions related to genetic regulation of hereditary cancer.